The present invention relates to a plasma processing apparatus for subjecting a target object, such as a semiconductor wafer, to a plasma process in an airtight vacuum process chamber, and more particularly to an improvement of an electrostatic chuck for holding the target object on a worktable arranged in the process chamber.
As an electrostatic chuck used in recent years, such a structure is known that is constituted by an insulating member made of, for example, ceramic, and a conductive film made of, for example, tungsten and disposed in the insulating member. When a predetermined DC voltage is applied to the conductive film, Coulomb force and Johnsen-Rahbeck force are generated on the insulating member, so that a semiconductor wafer is attracted and held thereby on the chucking surface of the electrostatic chuck.
The electrostatic chuck using ceramic as an insulating body as described above is prepared as an integratedly formed article having a diameter almost the same as that of the wafer, and then is attached to the mount surface of a worktable. For example, the electrostatic chuck is fabricated by sandwiching a conductive film of tungsten between a pair of ceramic plates, and heating them at a high temperature to bond them. When the heat treatment is performed, the electrostatic chuck is greatly shrunk and generates thermal stress, whereby the chuck may be distorted. Once the distortion is caused, the resultant shape remains after the electrostatic chuck is cooled down and never recovers the original shape given before the heat treatment. Accordingly, if the electrostatic chuck causes the distortion, the chuck is subjected to a polishing treatment after the heat treatment, so that it has a uniformly flat surface.
In recent years, ultra fine processing techniques have been developed to a great extent, and make it possible to perform a uniform process even on a large wafer, for example, of 12 inches or more. However, a large electrostatic chuck for such a large wafer tends to be distorted due to thermal stress caused by heat, for example, in a plasma process. This kind of distortion of an electrostatic chuck due to thermal stress becomes more prominent as the chuck size is increased. When an electrostatic chuck is distorted, so is a wafer thereon, thereby hindering the wafer from being uniformly processed. Further, even where a wafer of less than 12 inches, which was not so influenced by distortion of an electrostatic chuck, is processed by a recent ultra fine processing technique, the influence of the distortion sometimes cannot be ignored under severe conditions.
On the other hand, when an electrostatic chuck is heat treated to bond it to the mother body, i.e., a work table, in a process of manufacturing the chuck, the chuck tends to be more easily distorted due to thermal stress caused by difference in thermal expansion between the mother body and the chuck, as its size is increased. As described above, where some unevenness is formed on the chucking surface of the chuck due to the heat treatment, the chucking surface is subjected to a polishing treatment and flattened. In this case, a difference in distance from its conductive film to the chucking surface is brought about between the center and the periphery of the electrostatic chuck. This difference results in a difference in attraction force to a wafer, between the center and the periphery of the electrostatic chuck, thereby raising a difficulty in uniformly holding the wafer. Besides, where an electrostatic chuck greatly distorted is subjected to a polishing treatment, its conductive film may be exposed.
Further, a large electrostatic chuck requires a large manufacturing system which is contrary to present demands for compact facilities. A large electrostatic chuck also requires a large insulating member and a large conductive film, which entail complicated operations and high costs in their manufacturing process and transportation.